Driver protection circuit for preventing damage due to line contact with ground or supply voltage

ABSTRACT

This circuit is inserted between a terminal (RGB) and a line (LI). The signal of the input terminal (RGB) is applied to the base of a bipolar amplifier transistor (T 2 ) mounted with a top load (LR 1 ) and a bottom load (LR 2 ). A series transistor (T 1 ) mounted in series in the main signal path has its collector wired up as an electrode for receiving the input signal, its emitter feeds a line (LI), and its base is linked by a resistor to the collector of the second transistor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a protection circuit to be insertedinto an analog signals transmission line, circuit furnished with asupply terminal, an earth terminal, an input terminal and an outputterminal, circuit comprising a first so-called series transistor, acurrent-receiving electrode of which is linked to a terminal to which isapplied a voltage substantially equal to the voltage of the inputterminal whilst a current-delivery electrode of this transistor feeds anoutput terminal, and a second transistor for controlling the first.

2. Description of the Related Art

A protection circuit is known from the document DE-OS-28 12 157.According to this document, a switching transistor inserted into a lineis associated with an auxiliary transistor switched on by a signal atthe terminals of a resistor for measuring the current, which addscurrent into the base of the switching transistor.

An object of the invention is to make the circuit still more efficient.

SUMMARY OF THE INVENTION

For this purpose, the second transistor is mounted as an amplifier, witha current-receiving electrode loaded by a so-called top load elementlinked to the supply terminal, a current-delivery electrode loaded by aso-called bottom load element linked to earth, and a control electrodewired up in such a way that the voltage of the input terminal issubstantially applied thereto, whilst a control electrode of the seriestransistor is linked to the second transistor on the top load elementside.

Particular modes of implementation of the invention appear in thedependent claims 2 to 6.

BRIEF DESCRIPTION OF THE DRAWINGS

These aspects of the invention as well as other more detailed aspectswill be more clearly apparent by virtue of the following description ofan embodiment constituting a non-limiting example.

FIG. 1 represents a road vehicle.

FIG. 2 is a diagram of an embodiment of the circuit, designed to beplaced between an analog signal generator and a line for transmittingthis signal.

FIG. 3 is a diagram of an embodiment of the circuit, designed to beplaced between a line for transmitting a signal and an apparatus.

FIG. 4 is a diagram of a variant of use of the circuit of FIG. 3.

DETAIL DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The vehicle 1 of FIG. 1 is furnished with a central unit which generatesan analog signal and with a line for transmitting this signal whichculminates at a peripheral unit incorporated in the dashboard DB of thevehicle. The central unit CU is for example the computer of a navigationapparatus which yields color component signals RGB for a display, theline is for example a shielded bipolar line LI (one per color), and theperipheral unit is for example a liquid crystal display DISP.

The circuit of FIG. 2 is supplied by a voltage VCC of for example 12volts. It comprises an RGB input terminal, to which the computer CUdelivers a signal, and an output terminal linked to the line LI. A firstso-called series transistor T1, of NPN type, is placed in such a waythat its main current path (going from the collector to the emitter)links the input terminal to the output terminal via various elementswhich will be described later.

The signal of the input terminal is applied to a preamplifier stageknown per se, consisting of a transistor PT mounted in follower modetogether with a load resistor R1. The signal from the preamplifier stagePT is applied to the control electrode, namely the base, of a secondtransistor T2, of the same conduction type as the series transistor,mounted also as a conventional amplifier with a current-receivingelectrode, namely the collector, loaded by a so-called top load element,namely the resistor LR1 linked to the supply terminal, and acurrent-delivery electrode, namely the emitter, loaded by a so-calledbottom load element, namely the resistor LR2, linked to earth.

The first transistor T1 possesses a current-receiving electrode, namelyits collector, wired up to the emitter of the second transistor T2, thatis to say on the bottom load element LR2 side. The current-receivingelectrode of T1 is thus linked to the terminal 3 to which is applied avoltage substantially equal to the voltage of the input terminal. Acurrent-delivery electrode, namely its emitter, feeds the line LI, and acontrol electrode, namely its base, is linked by a resistor R3 to thecollector of the transistor T2, that is to say on the top load elementside. The main current in T1 is delivered from VCC via the resistor LR1.If for example the line LI were short-circuited with earth, an excessivecurrent in LR1 would cause the voltage at the point 2 to drop, togetherwith the base voltage of T1, thus limiting the current delivered to theline.

A diode D is wired up between the line LI and the base of the firsttransistor T1, with its cathode linked to the base of T1, hence in asense such that it is disabled in normal service. It would have adisabling effect if the line LI were accidentally to come into contactwith the 12-volt supply. A small model is sufficient since a heavycurrent never passes therethrough, owing to the fact that it is inseries with R3. The main current path of the first transistor T1 isshunted by a capacitor C2 between its collector and its emitter, so asto short-circuit the dynamic resistance of this transistor. A low-passfilter consisting of an inductor L in series, and of two capacitors C1in parallel is inserted in the current path going from the inputterminal to the output terminal.

An exemplary embodiment employs the following values: R1=470 ohms, LR1and LR2=33 ohms, R3=4700 ohms, C1=1 nF, L=1 μH, D=1N4148.

The circuit of FIG. 3 is supplied by a supply VCC, of 12 volts forexample. It is furnished with an input terminal linked to the line LIand with an output terminal RGB, which is for example an input terminalof an apparatus.

The main current path of a first so-called series transistor T1, of NPNtype, links the input terminal of the circuit to its output terminal.The voltage on the input terminal (LI) is applied via a resistor R4 tothe control electrode, namely the base, of a second transistor T2 of NPNtype, mounted as a comparator between a reference voltage developed atthe terminals of a so-called bottom load and applied to its emitter, andthe voltage of its base. The bottom load is a Zener diode DZ, forexample a 5-volt model, linked to earth and through which a resistor R2causes a current from the supply VCC to pass. The collector of thetransistor T2 is linked to the supply VCC by a so-called top load LR1.The base of the series transistor T1 is linked to the collector of T2 bya resistor R3; it could also be linked to it directly.

The input of the setup comprises a low-pass filter consisting of aninductor L in series and of a capacitor C1 in parallel. This filterensures protection against electromagnetic interference. The maincurrent path of the transistor T1 is shunted by a capacitor C2, so as toshort-circuit the dynamic resistance of this transistor. The values ofthe elements may be determined from those of the corresponding elementsof the setup of FIG. 2.

If the line LI is accidentally short-circuited with earth, thetransistor T2 is disabled, and a current passes only through the linevia the forward-biased base-collector junction of the transistor T1;this current is limited by the resistor LR1 and possibly by R3.

If the line LI is accidentally short-circuited with the supply VCC, thissaturates the transistor T2, thereby maintaining its collector voltagevery close to the 5-volt Zener voltage, thus limiting the voltage on theload LR2 to around 5 volts.

The Zener diode DZ could be replaced by a simple resistor for the sakeof simplicity; the reference voltage would be less well defined but thesetup would remain efficient.

The protection circuit of FIG. 4 is placed between several signaltransmission lines, namely three lines LI-R, LI-G, LI-B, and severalinputs, for example three color signal inputs R, G, B, of the liquidcrystal display DISP. The circuit comprises several first transistorsT1R, T1G, T1B, whose control electrodes (bases) are linked on eachoccasion by a resistor to the output of the second transistor T2, thatis to say to its collector. The control electrode (base) of the secondtransistor T2 is linked to the lines by three star-configured branchesBR, BG, BB, each comprising a resistor.

The bipolar transistors could possibly be replaced by field-effecttransistors. In this case, the control electrode would be referred to asthe gate, the current-receiving electrode as the drain, and thecurrent-delivery electrode as the source.

KEY TO FIGURES

FIGS. 2 and 3:

RVB→RGB

FIG. 4:

BV→BG

T1V→T1G

V→G

What is claimed is:
 1. A protection circuit comprising: a supplyterminal, an earth terminal, an input terminal which receives a voltagesignal and an output terminal; a first series transistor, acurrent-receiving electrode of which is linked to a node which receivesa voltage substantially equal to the voltage of the input terminal; acurrent-delivery electrode of the first series transistor coupled to theoutput terminal, and a second transistor for controlling the firstseries transistor; wherein the second transistor is an amplifier, with acurrent-receiving electrode loaded by a top load element linked to thesupply terminal, a current-delivery electrode loaded by a bottom loadelement linked to the earth terminal, and a control electrode receivinga voltage that is substantially equal to the voltage of the inputterminal; and a control electrode of the first series transistor linkedto the second transistor on the top load element side, wherein the firstseries transistor conducts current between its current deliveryelectrode and current receiving electrode unless the voltage from itscontrol electrode drops from excessive current from the output terminal.2. The protection circuit as claimed in claim 1, connected between ananalog signal generator and a signal line for transmitting this signal,wherein the electrode of the second transistor which is loaded by thebottom load is linked to the output terminal via the series transistor.3. The protection circuit as claimed in claim 2, wherein a diode iswired up between the line and the control electrode of the seriestransistor, biased such that it is normally disabled.
 4. The protectioncircuit as claimed in claim 1, placed between a line for transmitting ananalog signal and an apparatus, wherein the bottom load element is aZener diode or a resistor bridge between the supply terminal and earth.5. The protection circuit as claimed in claim 4, placed between severaltransmission lines and the apparatus, and further comprising severalinput terminals, the control electrode of the first transistor beinglinked to the input terminals by several branches, and several secondtransistors each linked to an output terminal.
 6. The protection circuitas claimed in claim 1, wherein a main current path of the seriestransistor is shunted by a capacitor.
 7. A protection circuitcomprising: a supply terminal, an earth terminal, an input terminalwhich receives a voltage signal and an output terminal; a first seriestransistor, a current-receiving electrode coupled to the input terminal;a current-delivery electrode of the first series transistor coupled tothe output terminal, and a second transistor for controlling the firstseries transistor; wherein the second transistor is an amplifier, with acurrent-receiving electrode loaded by a top load element linked to thesupply terminal, a current-delivery electrode, and a control electrodereceiving a voltage that is substantially equal to the voltage of theinput terminal; and a control electrode of the first series transistorlinked to the second transistor on the top load element side, whereinthe first series transistor conducts current between its currentdelivery electrode and current receiving electrode unless the voltagefrom its control electrode drops from excessive current from the outputterminal.